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ZOOLOGY: A. H. STURTEVANT 555
in two days. This number of eggs therefore has not received the ex-
posure necessary to produce the change in crossing over. The culmina-
tion of the two days exposure is to be expected in those eggs so
situated that 125 to 175 eggs will be laid before them. Such an
interpretation makes it extremely, likely that the change in the amount
of crossing over is finally affected in the earliest oocytes, that is, at
the beginning of the growth period. The above evidence on the time
of applying the new temperature and the time when the change in
crossing over occurs, suggests that the crossing over process takes place
in the stage when the chromosomes of Drosophila are known to be finely
drawn out threads.
The decrease in the strength of linkage caused by temperature in no
way weakens the chromosome interpretation of linkage. It rather adds
to it considerable support, for it localizes the process of crossing over at
a period in oogenesis when twisting between homologous threads seems
possible. The evidence positively establishes the fact that crossing over
does not take place during the early oogonial divisions, and makes it
extremely improbable that it occurs at so late a stage in the growth
period as the thick thread stage favored by Janssens as the chiasmatype.
bridges, C. B., /. Exp. ZooL, Wistar Inst., Philadelphia, 19, No. 1, July, 1915.
2 Sturtevant, A. H., these Proceedings, 3, 1917, (555-558).
5 Cf. Snyder, C. D., Amer.J. Physiol., 22, 1908, (309).
GENETIC FACTORS AFFECTING THE STRENGTH OF LINKAGE
By A. H. Sturtevant
ZOOLOGICAL LABORATORY. COLUMBIA UNIVERSITY
Communicated by T. H. Morgan, July 5, 1917
In September, 1913, a wild female Drosophila of a stock from Liver-
pool, Nova Scotia, was crossed to a male bearing the second chromo-
some mutant characters vestigial and speck. A single daughter of this
mating was tested, and gave no crossovers among 99 offspring, though
vestigial and speck usually show about 37% crossing over. This
strain has since been bred in very large numbers, and the experiments
are being continued; but it has seemed advisable to report briefly on some
of the results obtained. 1
It has become clear that the original result was due to something in
the second chromosome derived from the Nova Scotia female. Two
of her granddaughters and all of her later descendants that were known,
from linkage, to have received the second chromosome in question gave
556 ZOOLOGY: A. H. STURTEVANT
similar results; but those descendants known not to have received it
gave the 'usual' result. This was true whether the chromosome
came from the mother (as above) or from the father. And when other
second chromosome genes were substituted for vestigial and speck, un-
usual ratios still appeared; but for any combination the result was
relatively constant, and the combined 'unusual' results give as con-
sistent a scheme as the 'usual' results. The first column of table 1
shows the percentage of crossing over in the ordinary females for various
combinations of second chromosome genes. 2 The second column gives
similar data for tests of females that carry a Nova Scotia chromosome.
Figures 1 and 2 are chromosome maps constructed on the basis of these
^ lr pr vO c Sp
A : 1 * F 1 I
j or ygc §p
o.i Q ~W
4L — 1 ) it:. .. r
on HZ ¥ Uti </«? So. I
Sir pr C Sp (
pr V ■>!
I H- -4
Fi 3 .4
$ t pr sp
A large number of these experiments have been made with females
having one Nova Scotia chromosome, and its mate bearing the mutant
genes black, purple, and curved. Numerous tests have been made in
order to determine the nature of the crossovers produced. It has been
found that those crossovers that receive the part of the original Nova
Scotia chromosome lying to the left of the purple locus still give unusual
results and transmit the peculiarity to all of their descendants that
receive this piece. The results produced by such females are not, how-
ever, the same as those produced when all the Nova Scotia chromosome
ZOOLOGY: A. H. STURTEVANT
is present, as is shown in column 3 of table 1 and in figure 3. When
the other piece of the Nova Scotia chromosome, lying to the right of
purple (and, in many experiments, that part of it that is also to the
left of speck) is tested, a still different result is obtained, as shown in
column 4 and figure 4.
PERCENTAGE OF CROSSING OVER
Cni C r i
Star curved ....
Black purple. . . .
Black curved. . .
Black speck. . . .
Purple curved . .
Purple speck.. . .
Vestigial speck. .
Curved speck. . .
f Total length. . .
* These few (6) crossovers are doubtful. None of them were tested; and there is apt to be
a small percentage of error in classifying Star flies.
f Calculated as Star black + black purple + purple curved + purple speck in all but the last
two columns, where purple speck is used. In no case further corrected for double crossing
It follows from these results that the original Nova Scotia chromosome
contained at least two factors causing reduced crossing over, and each
affecting chiefly the region in which it lies. We may call these two fac-
tors 'Cni' and 'Cur' (C for crossing over, after Muller, 3 II to dis-
tinguish them from similar factors affecting other chromosomes, I and
r for left hand and right hand).
By appropriate matings it has been possible to obtain females homo-
zygous for C nr , and a surprising result has been observed: the effect
produced by C nr when heterozygous disappears when it becomes
homozygous (see columns 5 and 6, figures 5 and 6). There can be no
doubt of this result, as it has been obtained repeatedly, and has often
been checked by tests of the offspring. It has also been paralleled more
recently by results obtained by Muller 3 (1916) and by me 4 with C ni .
Similar tests of homozygous C n i have not been possible since no chromo-
558 ASTRONOMY: F. H. SEARES
some has yet been obtained that contains both that factor and any
mutant factor within its "sphere of influence."
Backcross tests have shown that males of all the above types with
respect to C n i and C m resemble "normal" males in giving no cross-
overs at all.
Included in the above tables are a considerable number of data in-
volving three or more loci at once; and these agree with the table in show-
ing clearly that the linear order of the factors established for the usual
second chromosome is unchanged by the factors under discussion. The
amount of crossing over is altered, often markedly, and not usually
proportionately in different regions; but the factors keep their same
sequence. This result serves to emphasize the importance of consider-
ing the distances on chromosome maps as only diagrammatic, not as
representing actually proportionate distances between the genes, al-
though actual distance is evidently an important factor influencing the
end result. It does not, I think, in any way weaken the case for the
chromosome hypothesis, but merely shows, together with the results of
Bridges 5 (1915) and Plough 6 (1917), that any chromosome map is avail-
able for purposes of numerical prediction only when the conditions
under which it was made are duplicated.
1 Some of the early results were reported at the 1913 meeting of the American Naturalists,
and brief references have been published by me (1915) and by Muller (1916).
2 Much of this data has not hitherto been published. It has been collected mainly by
Dr. C. B. Bridges, to whom I am indebted for permission to use it.
3 Muller, H. J.;Amer. Nat, Lancaster, Pa., 50, 1916, (193, 284, 350, 421).
4 Sturtevant, A. H., Zs. Abst. Vererb., 13, 1915, (234).
e Bridges, C. B., J. Exp. Zool., Wistar Inst. Philadelphia, 19, 1915, (1).
6 Plough, H. H., these Proceedings, 3, 1917, (553-555).
FURTHER EVIDENCE ON THE CONCENTRATION OF THE STARS
TOWARD THE GALAXY
By Frederick H. Seares
MOUNT WILSON SOLAR OBSERVATORY. CARNEGIE INSTITUTION OF WASHINGTON
Communicated by G. E. Hale, July 9, 1917
In a previous communication 1 attention was directed to the striking
difference in the results for the distribution of the stars with respect to
the galactic plane found by Kapteyn 2 and by Chapman and Melotte. 3
An analysis of the counts of stars on photographs of 88 Selected Areas,
made at Mount Wilson with the 60-inch reflector, gave preliminary
values for the variation in the totals to magnitude 17.5 which agree well
with those of Kapteyn.